It is known that the wavelength the light output from a semiconductor laser is greatly varied by a temperature change of the semiconductor laser. Accordingly, in the semiconductor laser device, the temperature of a semiconductor laser is maintained to a predetermined temperature by a peltier element and the like so that the wavelength of an output light is stabilized (refer to, for example, Japanese Patent Application Laid-Open Publication No. 06-188503).
FIG. 7 shows an arrangement of a conventional external resonator type semiconductor laser device 100. The semiconductor laser device 100 has a power supply 10, a peltier element 11, a semiconductor laser 12, an optical waveguide 13, and a diffraction grating 14. The semiconductor laser device 100 operates as described below. A current is supplied from the power supply 10 to the semiconductor laser 12 and light is generated. Further, a current is supplied from the power supply 10 to the peltier element 11 so that the semiconductor laser 12 is kept to a predetermined temperature. The generated light is coupled with the optical waveguide 13 from an emission end face of the semiconductor laser 12 and transferred to the diffraction grating 14. The diffraction grating 14 acting as an external resonator reflects a part of light having a predetermined wavelength and resonates only the wavelength. In the semiconductor laser device 100, the wavelength of the light generated by the semiconductor laser 12 is stabilized by the peltier element 11, a wavelength selectivity of output light 70 is enhanced by the diffraction grating 14.    Patent Document 1: Japanese Patent Application Laid-Open Publication No. 06-188503
FIG. 8 shows a current-light output characteristics (IL characteristics) of the semiconductor laser device 100. Although a light output is increased by increasing a current, when the current exceeds a predetermined current value, the light output is made unstable (kink). It is assumed that this is because a longitudinal mode is made sensitive to disturbance as a result that the semiconductor laser is operated at the predetermined temperature as described above and thus the longitudinal mode shifts from single to multiple. In particular, since the kink of light intensity becomes noise in an image application device and the like, a problem arises in that it is difficult to make use of the semiconductor laser device 100 having the IL characteristics to the image application device and the like.
To solve the above problem, an object of the present invention is to provide a semiconductor laser device which operates stably in an entire current region and a driving method of the semiconductor laser device.